As a technique that accurately judges a road surface condition and controls a pressure of a compression force (a belt clamping force) of a belt type continuously variable transmission properly according to an actual road surface condition, it has been disclosed in Patent Document 1. More specifically, a detection value of a rotation speed (the number of revolutions) of a driving wheel is subjected to a band-pass filter process, and a value obtained by this band-pass filter process is subjected to time total integration, then the road surface condition is judged according to this time total integration value. In a case where the road surface condition is judged to be a no-good road, the compression force is set to be higher than that of a case where the road surface condition is judged to be a good road.
In the technique of the Patent Document 1, however, since the band-pass filter process of the detection value is performed and the time total integration is also performed in order to accurately judge the road surface condition, it takes time to judge the road surface condition. And, even if the no-good road can be judged, a control to increase the compression force is delayed, then there is a risk that a belt slip will occur. For instance, on a road whose road surface friction coefficient (hereinafter, called μ) is uneven, immediately after the driving wheel slips at a low μ part, the driving wheel grips the road at a high μ part, and a torque inputted to the continuously variable transmission side is increased. If it takes time for judgment of the no-good road on such road surface, the control to increase the compression force is delayed also due to response delay of the hydraulic pressure, then as a problem, the slip between the belt and a pulley will occur.